This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Evidence is emerging that innate immune responses can be exploited for therapeutic purposes such as the development of adjuvants for vaccines and the treatment of a wide range of diseases including asthma, infection, and cancer. An important concern of such therapies is, however, that over-activation of innate immunity may lead to the clinical symptoms of septic shock. Thus, an important issue for the design of safe immune modulators is a detailed knowledge of structure-activity relationships to harness beneficial effects without causing toxicity. Lipopolysaccharides (LPS) are structural components of the outer membrane of Gram-negative bacteria that offer great promise for the development of immuno-modulators. It has been demonstrated unequivocally that lipid A is the inflammation-inducing moiety of LPS. Recent structural studies have shown that the carbohydrate backbone, degree of phosphorylation, and fatty acid acylation patterns of lipid A vary considerably among bacterial species. These structural differences probably account for the highly variable in vivo and in vitro host responses to LPS. There is also some indication that structurally different lipid As may differentially induce proinflammatory responses. Heterogeneity in the structure of lipid A within a particular bacterial strain and possible contamination with other inflammatory components of the bacterial cell-wall complicates the use of either LPS or lipid A isolated from bacteria to dissect the molecular mechanisms responsible for the biological responses to specific lipid A molecules. To determine whether the structure of lipid A can modulate innate immunological responses, we have synthesized a panel of lipid As derived from E. coli and S. typhimurium LPS that differ in acylation and phosphorylation pattern, and lipid length. We have determined the potencies and efficacies of a wide range of mediators induced by the well-defined lipid As. It was found that cellular activation with a particular compound can give potencies (EC50 values) for various mediators that can differ by as much as 100-fold. Furthermore, particular modifications affected the potencies of cytokines differently. Collectively, our data show for the first time that structurally different lipid As can induce distinct immune responses. This information is of critical importance for the future development of immuno-therapies.